Method for preparing lithium pyrophosphate and application thereof

Abstract

The application relates to the technical field of lithium pyrophosphate preparation, and particularly provides a lithium pyrophosphate preparation method and application, which comprises the following steps: S1, mixing lithium carbonate with water to prepare a slurry, then slowly adding pyrophosphoric acid until the pH reaches 6.5-7.5, and filtering after reaction; S2, adding an alcohol organic solvent to the filtrate obtained in the filtering until lithium pyrophosphate is completely precipitated, and then filtering again; and S3, drying the solid product obtained in the filtering in step S2 to obtain a lithium pyrophosphate product. In the application, the pyrophosphoric acid is slowly added to the lithium carbonate slurry, lithium pyrophosphate is obtained through reaction, lithium carbonate and lithium phosphate are not soluble or slightly soluble in water, most of the by-products can be removed through filtering, then the lithium pyrophosphate is precipitated through an alcohol organic solvent, and a lithium pyrophosphate product with high purity, small and uniform particles is obtained, and the application of the lithium pyrophosphate product in the field of battery materials is significantly expanded.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium pyrophosphate preparation, and particularly relates to a lithium pyrophosphate preparation method and application. BACKGROUND

[0002] With the development of new energy vehicles, the market has higher and higher requirements for the endurance mileage, cycle performance and safety of lithium power batteries. As an important component of lithium ion power batteries, the positive electrode material, including lithium iron phosphate and ternary positive electrode material, plays a key role in the electrochemical performance, safety performance and cost of the whole battery.

[0003] Currently, the performance improvement research of the positive electrode material mainly has two directions: 1) surface coating, that is, dispersing the positive electrode material in a phosphate aqueous solution, then drying and high-temperature calcining to form a surface coating layer; and 2) doping, that is, doping metal or non-metal elements in the positive electrode material lattice. Among them, the material mainly used in the surface coating research is lithium pyrophosphate, which is a good conductor of lithium ion diffusion. For example, Chinese patent CN108511715A discloses a preparation method of lithium ion battery ternary material coated with lithium pyrophosphate. The pH of the ternary material raw material solution is adjusted to 11-12 by using lithium dihydrogen phosphate, and the precursor is obtained after reaction, solid-liquid separation, washing and drying. The lithium ion battery ternary material coated with lithium pyrophosphate is prepared by annealing the obtained precursor at 450-550 DEG C in an oxygen-containing atmosphere. This patent actually precipitates lithium phosphate in the form of lithium phosphate on the surface of the ternary positive electrode material under alkaline conditions, and then dries and high-temperature calcines to form a lithium pyrophosphate coating layer. However, lithium phosphate is stable under alkaline conditions and is difficult to dehydrate to form lithium pyrophosphate. For another example, Chinese patent CN102244241A discloses a preparation method of lithium pyrophosphate modified lithium iron phosphate composite material. Lithium pyrophosphate is used as a coating layer to modify the positive electrode material. Specifically, the lithium iron phosphate precursor, phosphorus source, lithium source and organic carbon source are ball-milled and uniformly mixed in a liquid system, and then dried and high-temperature calcined to form lithium pyrophosphate coated lithium iron phosphate positive electrode material. However, the process may cause uneven distribution of the phosphorus source and lithium source, resulting in side reactions.

[0004] In view of the role of lithium pyrophosphate in the modification of positive electrode materials and the fact that there is no report or sale of lithium pyrophosphate on the current market, it is necessary to provide an improved preparation method of lithium pyrophosphate to expand the application of lithium pyrophosphate. SUMMARY

[0005] The application aims to provide a preparation method and application of lithium pyrophosphate, which is obtained by slowly adding pyrophosphoric acid into lithium carbonate slurry, and then filtering most of the by-products, and then using alcohol organic solvent to precipitate lithium pyrophosphate, so as to obtain lithium pyrophosphate product with high purity, small and uniform particle size, and to significantly expand the application of lithium pyrophosphate in the field of battery materials.

[0006] To achieve the above-mentioned purpose, the application provides a preparation method of lithium pyrophosphate, which comprises the following steps:

[0007] S1, mixing lithium carbonate with water to prepare slurry, then adding pyrophosphoric acid dropwise until pH reaches 6.5-7.5, and filtering after reaction;

[0008] S2, adding alcohol organic solvent into the filtrate obtained in S1 until lithium pyrophosphate is completely precipitated, and then filtering;

[0009] S3, drying the solid product obtained in S2 to obtain lithium pyrophosphate product.

[0010] Pyrophosphoric acid and lithium carbonate are subjected to acid-base neutralization reaction in aqueous solution, and the reaction formula is as follows:

[0011] 2Li2CO3+H4P2O7=Li4P2O7+2H2O+2CO2↑

[0012] Lithium carbonate has small solubility in water, and is mixed with water to prepare slurry and subjected to liquid phase reaction with pyrophosphoric acid, so that the generated lithium pyrophosphate is dissolved in water, and the contact efficiency of the reaction raw materials is improved; after the reaction is completed, since lithium carbonate and one of the by-products, lithium phosphate (pyrophosphoric acid is partially hydrolyzed into phosphoric acid, and reacts with lithium carbonate to generate lithium phosphate), have small solubility in water, the one of the by-products, lithium phosphate, and unreacted lithium carbonate can be removed by filtration. Then, by taking advantage of the characteristics that lithium pyrophosphate is dissolved in water but not in alcohol organic solvents such as methanol or ethanol, the solubility of lithium pyrophosphate in the system is reduced by adding methanol and / or ethanol into the solution, so that the product is precipitated. The whole preparation process is simple and easy to operate, and the product has high purity, small and uniform particle size.

[0013] As a further improvement of the application, for every 100g of raw material lithium carbonate, the dropwise adding rate of pyrophosphoric acid is 4-6g / min, and if the reaction scale is expanded or reduced, the dropwise adding rate is expanded or reduced in the same proportion, for example, if the raw material lithium carbonate is 200g, the dropwise adding rate is 8-12g / min. By slowly adding pyrophosphoric acid, lithium carbonate is always in excess, which can immediately contact and react with pyrophosphoric acid to generate lithium pyrophosphate, and the generation of by-product lithium phosphate is minimized.

[0014] As a further improvement of the present invention, the mass ratio of lithium carbonate to water is 1:20-25. If the amount of water added is too small, the generated lithium pyrophosphate will precipitate when it reaches saturation, resulting in a significant loss. The present invention controls the liquid-solid ratio to ensure that the product lithium pyrophosphate is completely soluble in water, thereby improving the purity of the final purified product.

[0015] As a further improvement of the present invention, step S1 is performed at 20-30°C.

[0016] As a further improvement of the present invention, the alcohol organic solvent includes anhydrous ethanol and / or anhydrous methanol.

[0017] As a further improvement of the present invention, step S2 includes: cooling the filtrate to 10-15°C in an ice bath, adding an alcoholic organic solvent, stirring for 20-30 minutes, and then filtering; the amount of the alcoholic organic solvent is 0.1-1 times the volume of the filtrate. Cooling is more conducive to the precipitation of lithium pyrophosphate, thereby increasing the yield.

[0018] As a further improvement of the present invention, in step S1, after the pyrophosphate is added dropwise, stirring is continued for 10-15 minutes until the reaction is completed.

[0019] As a further improvement of the present invention, step S3 includes: adding an alcoholic organic solvent again to the solid product obtained by filtration in step S2, stirring and washing for 10-15 minutes, and then filtering and drying. Multiple precipitations of the alcoholic organic solvent can improve the yield, minimize the moisture content in the wet lithium pyrophosphate material before drying, thereby preventing agglomeration during drying and improving the uniformity of the lithium pyrophosphate product particle size.

[0020] As a further improvement of the present invention, the amount of alcohol organic solvent used in step S3 is 0.5-1.2 times the mass of the solid product.

[0021] As a further improvement of the present invention, the drying is carried out in a vacuum drying oven at 50-60°C and a vacuum degree of -0.08MPa for 6-8 hours to obtain lithium pyrophosphate product.

[0022] In some specific embodiments, the method for preparing lithium pyrophosphate includes:

[0023] 1) Take an appropriate amount of high-purity lithium carbonate and pure water to make a slurry at a solid-liquid ratio of 1:20-25. Add pyrophosphate to adjust the pH of the system to 6.5-7.5. After the reaction is completed, continue stirring for 10-15 minutes (in this reaction, lithium carbonate is in excess to ensure that the pH at the end of the reaction is above 6.5).

[0024] 2) Filter the above reaction solution (filtering to remove unreacted lithium carbonate and a small amount of generated lithium phosphate, the resulting solution is lithium pyrophosphate solution), place the filtrate in a beaker and stir, add 0.1-1 times (preferably 0.1-0.2 times) the volume of the filtrate of anhydrous ethanol or anhydrous methanol, after the addition is complete, cool in an ice bath to 10-15℃ and continue stirring for 0.5h, then filter;

[0025] 3) Wash the obtained wet material with 0.9-1.1 times its weight of anhydrous ethanol or anhydrous methanol for 10-15 minutes, and then filter.

[0026] 4) Dry the washed wet material in a vacuum drying oven at 50℃ and a vacuum degree of -0.08MPa for 8 hours to obtain lithium pyrophosphate.

[0027] The present invention also provides lithium pyrophosphate prepared by any of the above preparation methods. The purity of lithium pyrophosphate is greater than 98%, preferably greater than 99%.

[0028] The present invention also provides an application of the lithium pyrophosphate described above, wherein the lithium pyrophosphate is used in the preparation of ionic solid electrolytes, lithium iron phosphate material coating modifiers, ternary material coating modifiers, positive and negative electrode material conductive agents, positive electrode material lithium replenishing agents, negative electrode material pre-lithiation agents, positive or negative electrode material inorganic conductive adhesives or solid electrolyte interface agents.

[0029] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0030] 1. The reaction process is short, fast, simple to operate, and has high yield and purity with few byproducts;

[0031] 2. This invention can react at room temperature, resulting in low energy consumption;

[0032] 3. The methanol or ethanol used in this invention is environmentally friendly, inexpensive, and readily available;

[0033] 4. Lithium pyrophosphate precipitated from methanol or ethanol is a homogeneous precipitation, which can ensure the uniformity of product particle size. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention are described clearly and completely below. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0035] Example 1

[0036] 100g of high-purity lithium carbonate (99.99% purity, the same applies below, details omitted) was placed in a 3000ml beaker. Pure water was added at a liquid-to-solid mass ratio of 22:1, and the mixture was stirred while maintaining the temperature at 25±2℃. Pyrophosphate was then added dropwise to adjust the pH of the system to 7.5 at a rate of 5g / min. After the addition was complete, stirring was continued for 10min. The mixture was filtered to obtain approximately 2200ml of purified lithium pyrophosphate solution (temperature 22±2℃). This purified solution was placed in an ice bath and stirred. After the temperature dropped to 15℃, 200ml of anhydrous ethanol was added. A large amount of white insoluble matter was produced. Stirring was continued for 0.5h, and the mixture was filtered to obtain 186.5g of wet lithium pyrophosphate. This was washed with 190g of anhydrous ethanol for 10min, and then filtered to obtain 125.2g of wet lithium pyrophosphate. The wet lithium pyrophosphate was then placed in a vacuum drying oven at -0.08MPa and dried at 50℃ for 8h to obtain 87.6g of dry lithium pyrophosphate. The test results are shown in Table 1.

[0037] Example 2

[0038] 100g of high-purity lithium carbonate was placed in a 3000ml beaker, and pure water was added at a solid-liquid mass ratio of 1:22. The mixture was stirred while maintaining the temperature at 25±2℃. Then, pyrophosphate was added dropwise to adjust the pH of the system to 7 at a rate of 4g / min. After the addition was complete, stirring was continued for 10min. The mixture was then filtered to obtain 2200ml of purified lithium pyrophosphate solution (temperature 22±2℃). The purified solution was placed in an ice bath and stirred. After the temperature dropped to 15℃, 200ml of anhydrous ethanol was added. A large amount of white insoluble matter was produced. Stirring was continued for 0.5h, and the mixture was then filtered to obtain 211.6g of wet lithium pyrophosphate. The sample was washed with 200g of anhydrous ethanol for 10min, and then filtered to obtain 141.5g of wet lithium pyrophosphate. The wet lithium pyrophosphate was placed in a vacuum drying oven at -0.08MPa and dried at 50℃ for 8h to obtain 100.5g of dry lithium pyrophosphate. The test results are shown in Table 1.

[0039] Comparative Example 1

[0040] 100g of high-purity lithium carbonate was placed in a 3000ml beaker, and pure water was added at a solid-liquid mass ratio of 1:22. The mixture was stirred while maintaining the temperature at 25±2℃. Then, pyrophosphate was added dropwise to adjust the pH of the system to 5.5 at a rate of 5g / min. After the addition was complete, stirring was continued for 10min. The mixture was then filtered to obtain 2200ml of purified lithium pyrophosphate solution (temperature 22±2℃). The purified solution was placed in an ice bath and stirred. After the temperature dropped to 15℃, 200ml of anhydrous ethanol was added. A large amount of white insoluble matter was produced. Stirring was continued for 0.5h, and the mixture was then filtered to obtain 224.5g of wet lithium pyrophosphate. The sample was washed with 200g of anhydrous ethanol for 10min, and then filtered to obtain 147.8g of wet lithium pyrophosphate. The wet lithium pyrophosphate was placed in a vacuum drying oven at -0.08MPa and dried at 50℃ for 8h to obtain 108.7g of dry product. The test results are shown in Table 1.

[0041] Comparative Example 2

[0042] 100g of high-purity lithium carbonate was placed in a 3000ml beaker, and pure water was added at a solid-liquid mass ratio of 1:22. The mixture was stirred while maintaining the temperature at 25±2℃. Then, pyrophosphate was added dropwise to adjust the pH of the system to 7.5 at a rate of 5g / min. After the addition was complete, stirring was continued for 10min. The mixture was filtered to obtain 2000ml of purified lithium pyrophosphate solution (temperature 22±2℃). The purified solution was placed in an ice bath and stirred. After the temperature dropped to 15℃, stirring was continued for 0.5h. No solid precipitated. This process shows that at a certain concentration, the solubility of lithium pyrophosphate is less affected by temperature, and cooling alone cannot achieve the purpose of precipitating lithium pyrophosphate.

[0043] Comparative Example 3

[0044] The procedure is the same as in Example 1, except that the method of adding lithium pyrophosphate is changed from dropwise addition to a single addition. In this case, the reaction is vigorous, and the solution may boil and overflow the beaker, terminating the reaction. The reason this procedure is not feasible is that the system viscosity is high, the reaction is exothermic, and a large amount of carbon dioxide is produced. The gas cannot be released quickly after expansion, and there are many byproducts, which affect the formation of lithium pyrophosphate.

[0045] Comparative Example 4

[0046] The reaction process was the same as in Example 1, except that after obtaining 2200 ml of purified lithium pyrophosphate solution, it was directly evaporated and crystallized. After evaporating 500 ml of water, the solution was filtered, and the resulting material was subjected to a solubility test. The test procedure was as follows: 20 g of wet material was taken, 100 g of pure water was added, and the mixture was stirred at room temperature for 10 min, then filtered. The mass of the filter residue was 18.7 g. The results showed that lithium pyrophosphate degraded into other phosphates with lower solubility, such as lithium phosphate and lithium metaphosphate, during the evaporation and crystallization process. This contradicts the original intention of this invention, therefore the evaporation and crystallization scheme is unsuitable.

[0047] The purity of this invention is determined by the phosphorus and lithium content in the product. The particle size of the product was detected by XRD, and the lithium content was detected by ICP. The detection methods for pyrophosphate, hydrogen pyrophosphate, and dihydrogen pyrophosphate were the same as those for sodium pyrophosphate anion in food additives (GB 1886.339-2021, titration of pyrophosphate with zinc sulfate heptahydrate, and titration of hydrogen pyrophosphate and dihydrogen pyrophosphate with sodium hydroxide). The results are shown in Table 1. As the pH of the system decreases, a small amount of lithium hydrogen pyrophosphate is generated, leading to a decrease in the lithium content of the product. Therefore, this invention, by controlling the dropping process and pH value, can obtain a lithium pyrophosphate product with high purity.

[0048] Table 1. Dry basis test results of the examples and comparative examples.

[0049]

[0050] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing lithium pyrophosphate, characterized in that, Includes the following steps: S1. Mix lithium carbonate with water to form a slurry, then add pyrophosphate dropwise until the pH reaches 6.5-7.

5. After the reaction is complete, filter the mixture. S2. Add an alcoholic organic solvent to the filtrate until lithium pyrophosphate is completely precipitated, and then filter. S3. Dry the solid product obtained from filtering in step S2 to obtain lithium pyrophosphate product.

2. The method for preparing lithium pyrophosphate according to claim 1, characterized in that, The dropping rate of the pyrophosphate is 4-6 g / min.

3. The method for preparing lithium pyrophosphate according to claim 1, characterized in that, The mass ratio of lithium carbonate to water is 1:20-25; step S1 is carried out at 20-30℃.

4. The method for preparing lithium pyrophosphate according to claim 1, characterized in that, The alcoholic organic solvents include anhydrous ethanol and / or anhydrous methanol.

5. The method for preparing lithium pyrophosphate according to any one of claims 1-4, characterized in that, Step S2 includes: cooling the filtrate to 10-15°C in an ice bath, adding an alcoholic organic solvent, stirring for 20-30 minutes, and then filtering; the amount of the alcoholic organic solvent is 0.1-1 times the volume of the filtrate.

6. The method for preparing lithium pyrophosphate according to claim 1, characterized in that, In step S1, after the pyrophosphate is added dropwise, stirring is continued for 10-15 minutes until the reaction is complete.

7. The method for preparing lithium pyrophosphate according to claim 1, characterized in that, Step S3 includes: adding an alcoholic organic solvent again to the solid product obtained by filtration in step S2, stirring and washing for 10-15 minutes, and then filtering and drying.

8. The method for preparing lithium pyrophosphate according to claim 7, characterized in that, The amount of alcohol-based organic solvent used in step S3 is 0.5-1.2 times the mass of the solid product; And / or, the drying is carried out in a vacuum drying oven at 50-60°C and a vacuum degree of -0.08MPa for 6-8 hours to obtain lithium pyrophosphate product.

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